1. Field of the Invention
The invention relates to a magnetic-inductive flow meter with at least one measuring tube, at least one magnetic circuit device for implementing a magnetic circuit, and at least two electrodes for detecting a measurement voltage. The measuring tube has an inflow section, a measurement section, which adjoins the inflow section, and an outflow section, which adjoins the measurement section. A flow cross section of the measurement section is both smaller than an inlet-side flow cross section of the inflow section and smaller than an outlet-side flow cross section of the outflow section. The electrodes are located on or in opposite electrode sections in the measurement section of the measuring tube. Moreover, the invention also relates to a measuring tube for a magnetic-inductive flow meter.
2. Description of Related Art
The measurement engineering foundation for flow rate measurement with a conventional magnetic-inductive flow meter uses a measurement tube of a nonmagnetic material, for example, of a plastic or a nonmagnetic metal. The measuring tube is on a flow side in the region of the magnetic field generated by a magnetic circuit device. The measuring tube is not electrically conductive or is insulated electrically from the measurement fluid by an insulating lining. In operation, the magnetic field generated by the magnetic circuit device permeates the measuring tube at a measurement section in a direction that is essentially perpendicular to the flow direction. If a measurement fluid with a minimum electrical conductivity is flowing through the measuring tube, charge carriers in the conductive measurement fluid are deflected by the magnetic field. The charge carriers create an electrical potential difference on electrodes which are located perpendicular to the magnetic field and to the flow direction. The charge carriers are detected with a measurement device and are measured as a voltage. The measured voltage is proportional to the flow velocity of the charge carriers which are moved with the measurement fluid such that the flow rate in the measuring tube can be deduced from the flow velocity.
The sensitivity of the magnetic-inductive flow meter and the accuracy of the measurement which can be taken with the magnetic-inductive flow meter depend, among other things, on the magnetic field, which is generated with the magnetic circuit device in the region of the measurement section of the measuring tube, the geometry of the measurement section and the arrangement of the electrodes. The geometry of the arrangement relates to the homogeneity of the magnetic field produced in the region of the measurement section, the flow conditions of the measurement fluid in the measurement section and, thus, also the electrical field generated by the charge separation in the measurement section, which is the basis for the measurement. The tuning of these different components of the magnetic-inductive flow meter to one another is crucial to attain accurate measurements.
Varying the cross section of the measuring tube beyond its longitudinal extension and, therefore beyond its extension in the flow direction is known from the conventional art. The inlet-side flow cross section of the inflow section conventionally has the geometry of the process connection. Therefore, conventionally, a circular flow cross section having the nominal width of the pipe in the process is connected to the magnetic-inductive flow meter. The corresponding applies to the outlet-side flow cross section of the outflow section, which likewise faces the process and which can be connected to the process. When “flow cross section” is addressed here, it always means the free cross sectional area of the measuring tube which has been measured perpendicular to the flow direction and which is available to the flow, and, therefore without the wall thickness of the measuring tube at the pertinent site.
German Patent Application 10 2008 057 755 A1, which corresponds to U.S. Pat. No. 8,286,503 B2, for example, discloses that a flow cross section of an inlet-side end of an inflow section decreases toward a measurement section and an outlet-side flow cross section of the measurement section increases, in turn, to the outlet-side flow cross section of the outflow section of a measuring tube. The change of the cross section has the advantage that the flow velocity of the measurement fluid is increased in the region of the measurement section and, accordingly, a greater effect is also achieved for the charge separation as a result of the magnetic field in the measurement section.
The variable cross sectional geometry of the measuring tube beyond its longitudinal extension is achieved in the conventional art by comparatively complex production techniques, for example by casting a corresponding metal measuring tube, by internal high pressure forming or by injection molding of a plastic measuring tube. The production effort and the associated costs have, for a long time, prevented the use of magnetic-inductive flow meters for low cost, mass applications, for example as domestic water meters. This is due not only to the production costs associated with the measuring tube, but also to the altogether comparatively demanding hardware and measurement-engineering structure of a magnetic-inductive flow meter.